Magnetic resonance systems used for medical diagnostic purposes comprise a magnet for providing a strong homogeneous static magnetic field. The magnet is large enough to receive a patient therein. Radio frequency (RF) coils are provided in such systems to transmit RF pulses toward spins aligned with the static magnetic field. The RF pulses at the Larmor frequencies are transmitted into the RF coils of such systems to nutate or "tip" the spins into a plane orthogonal to the large static magnetic field. The spins in the orthogonal plane generate free induction decay (FID) signals or depending on the scan sequence, echo signals. It is the FID and/or echo signals that are used for diagnostic purposes.
The signals received from the spins in the patient are very small. To improve the signal-to-noise ratio, many different types of surface coils; i.e., coils that are placed adjacent to the skin surface of the patient are used. These coils in general are used merely for receiving purposes. Thus, the body coils that are used for transmitting must be quickly decoupled after having generated an RF magnetic field so as not to disturb the detection of the generated spin resonance signals by the surface coils.
To this end, it is well known in the prior art to use semi-conductor switches such as diodes in the RF body coil to decouple the body coil. A particularly useful body coil is the one disclosed in U.S. Pat. No. 4,757,290 issued Jul. 12, 1988. It is well known to place the diodes at zero crossing points in the conductors of the RF body coil so that the semi-conductor switches are operable with low voltage.
Control circuitry is generally used to control the body coil so that no transmitted signal is sent while the diodes are turned off responsive to signals from decoupling control circuitry. It is known that if the RF pulses are transmitted while the diodes are turned off, the diodes are liable to "burn out".
One of the problems of prior art circuitry for controlling the decoupling of the body coils, i.e., for switching the body coil diodes on and off is that transients, such as transients which may be caused when a gradient coil malfunctions have been known to switch the diodes to the off condition even when the control circuit has the diodes biased to the "on" condition. Subsequently, RF pulses placed across the diodes can seriously damage the diodes.
Thus, it is an object of the present invention to provide control circuitry for selectively decoupling body coils and for preventing the transmission of RF pulses whenever the body coil is decoupled; i.e., whenever the diode switches are turned off regardless of whether they are turned off due to high transients or turned off responsive to the signals from the decoupling control circuitry.